Enantioselective synthesis of axially chiral 1-arylisoquinolines by rhodium-catalyzed [2+2+2] cycloaddition

Article abstract of DOI:10.1002/chem.201003134

It's all in the application! An enantioselective synthesis of axially chiral 1-arylisoquinolines has been achieved, which involves a rhodium-catalyzed [2+2+2] cycloaddition of diphenylphosphinoyl-substituted 1-arylisoquinolines (see scheme). The new diphenylphosphinoyl-substituted axially chiral 1-arylisoquinolines were successfully derivatized to the corresponding axially chiral P,N ligand and Lewis base catalyst.

Full text of DOI:10.1002/chem.201003134

DOI: 10.1002/chem.201003134
Enantioselective Synthesis of Axially Chiral 1-Arylisoquinolines by
Rhodium-Catalyzed [2+2+2] Cycloaddition
Norifumi Sakiyama,[a] Daiki Hojo,[a] Keiichi Noguchi,[b] and Ken Tanaka*[a]
Axially chiral 1-arylisoquinoline and 2-arylpyridine deriv-
atives are valuable compounds as chiral ligands[1] and cata-
lysts[2] for various asymmetric catalytic reactions. However,
their conventional syntheses are based on the optical resolu-
tion of racemic compounds.[1,2] For example, the axially
chiral 1-arylisoquinoline-based P,N ligand quinap was syn-
thesized by optical resolution of its racemate with a stoichio-
metric amount of a chiral palladium(II) complex.[1d,f,j] The
axially chiral 1-aryl-phthalazine-based P,N ligand pinap, was
synthesized by separation of two atropisomeric diastereo-
mers derived from a chiral secondary alcohol or amine.[1k]
Axially chiral 1-arylisoquinoline-N-oxide (quinox) was syn-
thesized by optical resolution of the corresponding 1-aryliso-
quinoline by using a stoichiometric amount of chiral 1,1’-bi-
2-naphthol.[2c,d]
In 2004, Gutnov, Heller, and co-workers reported a spec-
tacular approach to axially chiral 2-arylpyridines. They de-
veloped the chiral cobalt(I)-complex-catalyzed enantioselec-
tive [2+2+2] cycloaddition of aryldiynes with nitriles, to
give axially chiral 2-arylpyridines.[3–8] However, this method
requires low reaction temperatures to gain high enantiose-
lectivity, and further reaction steps to introduce a phospho-
rus substituent. Recently, Clayden and co-workers reported
the first asymmetric synthesis of a quinap ligand by the dy-
namic thermodynamic resolution method,[9] whereas catalyt-
ic enantioselective synthesis has not been realized to date.
Conversely, our research group and others reported the
enantioselective synthesis of axially chiral biaryl phosphorus
compounds by the cationic rhodium(I)/axially chiral biaryl
bisphosphine complex catalyzed atropselective [2+2+2] cy-
cloaddition of diynes with phosphorus-substituted arylal-
kynes.[10,11] Herein, we report the application of this method-
ology to the highly enantioselective synthesis of axially
chiral 1-arylisoquinolines, which involves the first catalytic
enantioselective synthesis of diphenylphosphinoyl-substitut-
ed axially chiral 1-arylisoquinolines.
We first investigated the reaction of trimethylsilyl-substi-
tuted 1-ethynylisoquinoline 2a with ether-linked 1,6-diyne
1a in the presence of the cationic rhodium(I)/(R)-binap
complex (20 mol%). Excess 1a (3 equiv) was employed due
to its rapid homo-[2+2+2] cycloaddition. Pleasingly, the re-
action proceeded at room temperature to give the desired
[2+2+2] cycloaddition product 3aa in good yield with ex-
cellent enantioselectivity (Table 1, entry 1). The choice of
biaryl bisphosphine ligands appeared to have a modest
impact on the product yields, but not on the product enan-
tiomeric excess (ee) values (Table 1, entries 1–4). Since the
use of H8-binap and solphos gave 3aa in high yields, the re-
action conditions were further optimized with these ligands
(Table 1, entries 5–8). The use of a catalyst loading of
10 mol% and solphos as the ligand, significantly decreased
both yield and ee value (Table 1, entry 5). An increase of re-
action temperature to 808C also failed to improve the yield
of 3aa (Table 1, entry 6). In contrast, although the use of
5 mol% RhI/H8-binap catalyst at room temperature was
[a] N. Sakiyama, D. Hojo, Prof. Dr. K. Tanaka
Department of Applied Chemistry, Graduate School of Engineering
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
Fax : (+81)42-388-7037
E-mail: tanaka-k@cc.tuat.ac.jp
[b] Prof. Dr. K. Noguchi
Instrumentation Analysis Center
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003134.
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Chem. Eur. J. 2011, 17, 1428 – 1432

COMMUNICATION
Table 1. Optimization of reaction conditions for rhodium-catalyzed enan-
tioselective [2+2+2] cycloaddition of 1a with 1-alkynylisoquinoline 2a.
methyl-substituted 1-ethynylisoquinolines (2d, entry 7)
could be used.
The reactions of diphenylphosphinoyl-substituted 1-ethy-
nylisoquinoline 2e with tosylamide-linked 1,6-diyne 1b also
proceeded by using a high catalyst loading (10 mol% Rh)
and an elevated temperature (808C); however, with binap,
segphos, and H8-binap as ligands, the product ee values were
low (Table 3, entries 1–3). Fortunately, by using solphos as a
Table 3. Rhodium-catalyzed enantioselective [2+2+2] cycloaddition of
1,6-diynes 1b and 1a with diphenylphosphinoyl-substituted 1-ethynyliso-
quinoline 2e.
Entry Ligand
Catalyst [mol%] T [8C] Yield [%][a] ee [%]
1
2
3
4
5
6
7
8
(R)-binap
(R)-segphos
(R)-H8-binap 20
(R)-solphos
(R)-solphos
(R)-solphos
(R)-H8-binap
(R)-H8-binap
20
20
RT
RT
RT
RT
RT
80
60
64
72
83
26
17
11
73
99 (À)
98 (À)
99 (À)
97 (À)
93 (À)
86 (À)
99 (À)
93 (À)
Entry
1
Ligand
3
Yield [%][a]
ee [%]
20
10
10
5
1
2
3
4
1b
1b
1b
1b
1b
1a
(R)-binap
(+)-3be
(+)-3be
(À)-3be
(+)-3be
(+)-3be
(+)-3ae
86
36
77
65
22
58
20
32
38
74
75
77
(R)-segphos
(R)-H8-binap
(R)-solphos
(R)-solphos
(R)-solphos
RT
80
5
5[b]
6
[a] Yield of the isolated product.
[a] Yield of the isolated product. [b] At RT in CH2Cl2, catalyst
(20 mol%).
sluggish (Table 1, entry 7), an increase in the reaction tem-
perature to 808C improved the reaction rate to give 3aa in
high yield with a high ee value (Table 1, entry 8).
ligand, the ee value was improved with a slight loss of the
product yield (Table 3, entry 4). This reaction was also at-
tempted at room temperature with a high catalyst loading
(20 mol%) however, the reaction rate was low (Table 3,
entry 5). Under the optimum reaction conditions (Table 3,
entry 4), the reaction of compound 2e with ether-linked 1,6-
diyne 1a also proceeded in moderate yield with a good ee
value (Table 3, entry 6).
The generality of the reaction was then examined as
shown in Table 2. Both ether-linked 1,6-diyne 1a (Table 2,
entry 1) and tosylamide-linked 1,6-diyne 1b (entry 2) were
suitable substrates for this process. In the case of 1b,
1.1 equivalent was sufficient because of its slow homo-[2+
2+2] cycloaddition. With respect to the substituents at the
alkyne terminus of 1-ethynylisoquinoline, not only trime-
thylsilyl-2a (Table 2, entries 1 and 2) but also isopropyl- (2b,
entries 3 and 4), n-butyl- (2c, entries 5 and 6), and methoxy-
For comparison, the reaction of 1a with 1-(trimethylsilyl-
ethynyl)naphthalene (2 f) with the RhI/rac-binap catalyst
was examined (Scheme 1). Because of the rapid homo-[2+
Table 2. Rhodium-catalyzed enantioselective [2+2+2] cycloaddition of
1a and 1b with alkynes 2a–2d.
Scheme 1. Rhodium-catalyzed reaction of diyne 1a with alkyne 2 f.
Entry
1
2 (R)
3
Yield [%][a]
ee [%]
1
1a
1b
1a
1b
1a
1b
1a
2a (SiMe3)
2a (SiMe3)
2b (iPr)
2b (iPr)
2c (nBu)
(À)-3aa
(À)-3ba
(+)-3ab
(À)-3bb
(+)-3ac
(À)-3bc
(+)-3ad
73
78
75
79
84
83
71
93
92
94
91
90
89
90
2+2] cycloaddition reaction of 1a, the cross-[2+2+2] cy-
cloaddition product 3af was not obtained. This result clearly
indicates that the nitrogen atom of the isoquinoline unit of
compound 2a is essential to promote the desired cross-[2+
2+2] cycloaddition with 1a. On the other hand, the reaction
of 1,6-diyne 1b with 1-(diphenylphosphinoylethynyl)naph-
thalene (2g) using the RhI/(R)-solphos catalyst gave the
2[b]
3
4
5
6
7
2c (nBu)
2d (CH2OMe)
[a] Yield of the isolated product. [b] Catalyst: 10 mol%.
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1429

K. Tanaka et al.
cross-[2+2+2] cycloaddition product 3bg in almost identi-
cal yield, although the ee value was lower than that of 3be
(Scheme 2 vs. Table 3, entry 5).
Scheme 3. Competition experiment between 2e and 2i.
pound 3be was obtained with higher yield than 3bi in the
competition experiment (Scheme 3). We anticipated that 1-
arylisoquinoline 3be is more coordinative than 1-aryl-8-
methylisoquinoline 3bi, which might inhibit the reaction.
Indeed, the addition of 3be (10 mol%) to the reaction of 1-
alkynyl-8-methylisoquinoline 2i and 1b significantly lowered
the yield of 3bi (Scheme 4).
Scheme 2. Rhodium-catalyzed reaction of diyne 1a with alkyne 2g.
To obtain 1-arylisoquinolines possessing a configurational-
ly more stable biaryl axis, the reactions of 1-alkynyl-8-meth-
ylisoquinolines with 1,6-diynes were examined (Table 4). In-
terestingly, an 8-methyl substitution is beneficial; 8-methyl-
Table 4. Comparison of reactivity and enantioselectivity between 8-meth-
ylisoquinolines 2h and 2i and isoquinolines 2a and 2e.
Scheme 4. Rhodium-catalyzed [2+2+2] cycoaddition of 1b with 2i in
the presence of (+)-3be.
Entry
1
2 (R1, R2)
3
Yield[%][a]
ee [%]
1
2
3
4
1b
1a
1a
1b
1a
1a
1b
1a
2h (SiMe3, Me)
2h (SiMe3, Me)
2a (SiMe3, H)
2i [P(O)Ph2, Me]
2i [P(O)Ph2, Me]
2e [P(O)Ph2, H]
2i [P(O)Ph2, Me]
2i [P(O)Ph2, Me]
(À)-3bh
(À)-3ah
(À)-3aa
87
80
73
88
90
30
83
87
99
99
93
95
94
33
95
95
Derivatization of axially chiral 1-arylisoquinoline 3 into
chiral P,N ligand 4 and chiral isoquinoline-N-oxide 5 was
readily accomplished. Reduction of phosphine oxide 3be
with HSiCl3 and Et3N proceeded at 808C in good yield,
whereas the product phosphine 4be did not possess stable
axial chirality at room temperature (Scheme 5).[13] Fortu-
(+)-3bi
5[b]
6[b]
7[c]
8[b,c]
(R)-(+)-3ai
(+)-3ae
(+)-3bi
(R)-(+)-3ai
[a] Yield of the isolated product [b] Ligand: (R)-binap. [c] At RT in
CH2Cl2, catalyst (20 mol%).
isoquinolines 3bh and 3ah were obtained with higher yields
and ee values than those of isoquinoline 3aa (Table 4, en-
tries 1 and 2 vs. entry 3). This positive substituent effect was
also found in the reactions of diphenylphosphinoyl-substitut-
ed 1-ethynyl-8-methylisoquinoline 2i with 1b and 1a; the
desired cycloaddition products 3bi and 3ai were obtained
with significantly higher yields and ee values than those of
isoquinoline 3ae, by using 5 mol% of the rhodium catalysts
at 808C (Table 4, entries 4 and 5 vs. entry 6). Furthermore,
biaryl phosphine oxides 3bi and 3ai were obtained at room
temperature by using 20 mol% of the rhodium catalyst (en-
tries 7 and 8). The absolute configuration of axially chiral
biaryl phosphine oxide (+)-3ai was unambiguously deter-
mined to be R by the anomalous dispersion method.[12]
Scheme 5. Synthesis of axially chiral P,N ligands 4be and (+)-4bi.
nately, reduction of phosphine oxide 3bi proceeded under
the same reaction conditions to give the corresponding
phosphine 4bi, with stable axial chirality even at 808C, in
good yield and without racemization (Scheme 5). Oxidation
of 1-arylisoquinoline 3bi with meta-chloroperbenzoic acid
(mCPBA) also proceeded to give the corresponding N-oxide
5bi in good yield without racemization (Scheme 6).
To clarify the role of the 8-methyl substituent, a competi-
tion experiment between 2e and 2i was conducted. Al-
though 3bi was obtained with higher yield than 3be in sepa-
rate experiments (Table 4, entry 4 vs. Table 3, entry 3), com-
Finally, preliminary applications of the enantioenriched
P,N ligand 4bi and isoquinoline-N-oxide 5bi in asymmetric
catalysis were briefly examined. Since the previously report-
ed rhodium(I)/quinap-catalyzed asymmetric hydroboration
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Chem. Eur. J. 2011, 17, 1428 – 1432

Enantioselective Synthesis of Axially Chiral 1-Arylisoquinolines
COMMUNICATION
sponding axially chiral P,N ligand and Lewis base catalyst.
Future studies will focus on further optimization of chiral
ligand/catalyst structures and their applications in various
asymmetric catalyses.
Experimental Section
Scheme 6. Synthesis of axially chiral 1-arylisoquinoline-N-oxide (+)-5bi.
General procedure (Table 4, entry 4): Under an Ar atmosphere, (R)-H8-
binap (4.7 mg, 0.0075 mmol) and [RhACHTNUGTRNEUNG(cod)2]BF4 (3.0 mg, 0.0075 mmol)
of 2-chlorostyrene proceeds with insufficient yield and enan-
tioselectivity,[14] the rhodium(I)-catalyzed asymmetric hydro-
boration of 2-chloro- and 2-bromostyrenes 6a and 6b was
investigated with (+)-4bi as a ligand. Although the ee
values were still moderate, the corresponding chiral secon-
dary alcohols 7a and 7b were obtained in high yields
(Scheme 7). The asymmetric allylation of benzaldehyde (8a)
was also examined by using (+)-5bi as a catalyst. The corre-
were dissolved in CH2Cl2 (1.0 mL) and the mixture was stirred at room
temperature for 30 min. H2 was added to the resulting solution in a
Schlenk tube. After stirring at room temperature for 1 h, the resulting so-
lution was concentrated to dryness. (CH2Cl)2 (0.5 mL) was then added to
the residue at room temperature, followed by a solution of 2i (55.1 mg,
0.150 mmol) and 1b (45.4 mg, 0.165 mmol) in (CH2Cl)2 (1.0 mL). The
mixture was stirred at 808C for 16 h. The resulting solution was concen-
trated and purified by preparative TLC (EtOAc) to give (+)-3bi
(84.6 mg, 0.132 mmol, 88% yield, 95% ee) as a pale yellow solid.
Acknowledgements
This work was supported partly by Grants-in-Aid for Scientific Research
(nos. 20675002 and 21·7999) from MEXT, Japan. We thank Takasago Int.
Co. for the gift of segphos and H8-binap, Solvias AG for the gift of sol-
phos under the University Ligand Kit program, and Umicore for gener-
ous support in supplying the rhodium complex.
Scheme 7. Rhodium-catalyzed asymmetric hydroboration with axially
chiral P,N ligand (+)-4bi.
Keywords: asymmetric catalysis
·
axial chirality
·
cycloaddition · isoquinolines · P,N ligand
sponding chiral secondary alcohol (R)-9a was obtained in
good yield with a moderate ee value (Scheme 8). Compara-
ble results are obtained by using binapo [2-diphenylphosphi-
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the synthesis of diphenylphosphinoyl-substituted arylisoqui-
nolines, by a rhodium-catalyzed [2+2+2] cycloaddition.
The new diphenylphosphinoyl-substituted axially chiral 1-ar-
ylisoquinolines were successfully derivatized to the corre-
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Received: November 1, 2010
Published online: January 5, 2011
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Chem. Eur. J. 2011, 17, 1428 – 1432